REFRACTORY INCLUSIONS IN THE PROTOTYPICAL CM CHONDRITE, MIGHEI

Refractory inclusions in the Mighei CM2 meteorite are very abundant, 3 5 being found in three thin sections of the meteorite. The most common are pyroxene-rimmed spinel-rich inclusions that occur in a range of s tructural and textural types. Some inclusions are simple aggregates of crystals or rimmed crystals, ranging from loose clusters of spinel gr ains to distended chains of pyroxene-rimmed spinel knots. Others are m ore compact and may have experienced recrystallization or melting; the se include ''nodular'' spinel inclusions with polygonal-granular-textu red spinel interiors and pyroxene rims, ribbons of spinel (in some cas es with hibonite and/or perovskite) that are again rimmed by pyroxene, and refractory spherules. Four inclusions are complex aggregates of s everal texturally distinct objects each. Collectively, Mighei inclusio ns are similar to those in Murchison but appear to be somewhat more al tered. Volatility fractionated and unfractionated trace element patter ns similar to those found in refractory inclusions in other carbonaceo us chondrites are represented in Mighei inclusions and, in addition, s ome Mighei inclusions have volatility fractionated pattern types not p reviously found in refractory inclusions. In three of the complex aggr egates, fundamentally different trace element patterns are present wit hin different portions of each inclusion, meaning that these objects a ccreted from genetically different components that never equilibrated with one another subsequent to accretion. Bulk inclusion magnesium iso topic compositions show minor mass fractionation effects: F-Mg values range from -2.3 to +7.5 parts per thousand, but the average F-Mg value is indistinguishable from normal, 1.8 +/- 1.2 parts per thousand. The re are no correlations between magnesium isotopic composition and text ural type or trace element pattern type. The diverse inclusion types i n Mighei represent a variety of origins and histories. Textures sugges t that the nodular and banded inclusions may have experienced solid-st ate recrystallization, whereas the porous and chain-like varieties and complex aggregates have not. Magnesium isotopic compositions and trac e element abundances suggest that the Mighei inclusions are unlikely t o be evaporation residues and are more likely to be the products of co ndensation or recondensation processes.